Multicell electrolytic furnace with suspended electrodes and method of aluminum production



DE GARAB MULTICELL ELECTROLYTIC FURNACE WITH SUSPENDED 4 Sheets-Sheet 1 Nov. 14, 1967 ELECTRODES AND METHOD OF ALUMINUM PRODUCTION Filed NOV. 1963 ll g FIG.I

Nov. 14, 1967 0. DE GARAB, 3,352,767

MULTICELL ELECTROLYTIC FURNACE WITH SUSPENDED ELECTRODES AND METHOD OF ALUMINUM PRODUCTION Filed Nov. 1963 4 Sheets-Sheet 3 3,352,767 MULTICELL ELECTROLYTIC FURNACE WITH SUSPENDED ELECTRODES AND METHOD OF 1963 Nov. 14, 1967 G 0. DE GARAB Is T C4 U D O R P. M U N I M U L A United States Patent 3,352,767 MULTICELL ELECTROLYTIC FURNACE WITH SUSPENDED ELECTRODES AND METHOD OF ALUMINUM PRODUCTION Giorgio Olah de Garab, Milan, Italy, assignor of seventy percent to Montecatini Edison S.p.A., and thirty percent to Giuseppe de Varda, both of Milan, Italy Filed Nov. 5, 1963, Ser. No. 321,577 Claims priority, application Italy, Nov. 10, 1962, 22,215/ 62 19 Claims. (Cl. 20467) ABSTRACT OF THE DISCLOSURE Furnace for electrolytic production of aluminum includes a vat for containing a fluoride bath of dissolved alumina, and a series of electrodes rigidly suspended in the vat including a plurality of terminal electrodes and at least one bipolar intermediate electrode between the terminal electrodes, consecutive electrodes of the series defining mutually spaced faces of contrary polarity. The bipolar intermediate electrode includes a permanent portion with a face having lateral and bottom edges, and a consumable portion having projections extending beyond a plurality of the edges of the faces of the permanent portion so that the electrolytically active area of the consumable portion is larger than the electrolytically active area of the opposed and confronting electrodic face of the next adja cent electrode of the series.

The present invention relates to a method and apparatus for producing aluminum by means of fused-bath electrolysis from alumina dissolved in a bath of molten salts.

This invention is an improvement in the type of furnace and electrodes described in the De Varda patents, for example U.S. Patent No. 2,952,592, and 3,029,194. In particular, the invention is an improvement of the processes and furnace described in Patent No. 3,178,363 of De Varda and Olah de Garab, issued Apr. 13, 1965.

The type of furnace involved is generally characterized by electrodes which are supported or suspended through or by insulating members located either entirely outside the furnace, or connected to the furnace only at parts which are not intended to be in contact with the molten electrolytic bath.

The inside surfaces of the furnace, and also the faces of the electrodes that are not intended to be electrolytically active, are normally coated, according to the prior art, with a refractory insulating material which is inert to the electrolytic bath, the electrodes being distinctly spaced from the inside of the furnace, at least up to a line corresponding to the highest possible level the bath may reach. No anodic part of the individual electrodes, designed to contact the electrolytic bath, is simultaneously in direct electric contact with either the cathodic part of another electrode or of the vat structure, there being no electrical connection or contact through the lining refractory, nor directly.

Such furnaces may comprise one or more vats, each vat containing stationary, vertical or subvertical electrodes of carbonaceous material or of graphite. The electrolysis space is between the faces of contrary polarity of consecutive electrodes. If more than two electrodes are used, the intermediate electrode or electrodes are bipolar elec trodes, having no attached current conductors, that is, during ordinary serial current flow between adjacent cells. The terminal electrodes are provided with fixed metallic current-carrying stubs which distribute the current substantially uniformly on the active surfaces of the elechce trodes. One series or a plurality of series of electrodes can be arranged as a closed necklace of cells, in the same furnace, provided with means for circulating the electrolytic bath about the electrodes in a series of vats. The electrodes may be supported or suspended individually, for example through or by members resting on the top of the furnace or vat. The bottom of the furnace vat may be shaped or formed so as to provide individual open channels or grooves extending across the vat, to collect and discharge the metal produced by electrolysis on the cathodic faces of the individual electrodes.

Heretofore, it has been customary to frame both the bipolar electrodes and the terminal monopolar electrodes with a protective refractory coating which is inert both to the bath and to the electrolysis. Such refractory frame consisted of side coatings, base coatings, and top coatings, as well as transverse coatings in the case of monopolar terminal electrodes. These coatings are normally fixed to the carbonaceous material of the electrode body, such as graphite, by means of dove-tailed joints, or by the application of a mastic on pitch base between the coating and the stationary permanent electrode.

These coverings of the prior art, consisting of frames of specially electrically insulated refractory, are customarily applied on the sides and bases of the suspended bipolar and monopolar electrodes, and are designed to make the covered surfaces inactive to the bath and to protect them from the electrolytic consumption at the surfaces intended to remain electrolytically inactive.

However, such coatings or coverings providing a special refractory frame represent a substantial cost. Moreover, the refractory hitherto employed in the zones adjacent to the active anodic surfaces have proved to be subject to a certain amount of chemical and electro-chemical attack, which in the long run could lead to corrosion of the refractory material.

It is an object of the present invention to make it possible to completely or partially eliminate such refractory layers on the suspended electrodes without adversely affecting the rate of corrosion of the electrodes by the bath.

It is another object of the invention to achieve a greater efiiciency of electrodes and electrolysis process in electrolytic furnaces without increasing the cost, and to obtain at the same time a remarkable saving in material as compared to the furnaces and electrode consumption of the prior art.

It is a further object of the invention to provide an electrolytic furnace for aluminum production which will be economical to operate, which has an installation cost remarkably lower than furnaces of the prior art, which has a'favorable ratio between anodic and cathodic active surfaces, and which results in a reduced cost of producing aluminum as compared to furnaces of the prior art.

To these ends, and in accordance with the main features of the present invention, a furnace for aluminum production through fused electrolysis from A1 0 is provided with a series of bipolar intermediate electrodes located between monopolar terminal electrodes. All of the electrodes are rigidly suspended from above and located inside a vat having an inert inner lining. A header of protective material may be provided over the electrodes, in which case the suscathode portion and a consumable anode portion confronting and spaced from the surface of an adjacent cathode. Of course also the terminal anode has a permanent portion in addition to its consumable anodic portion,

whilst the terminal cathode is entirely permanent. The

active areas of the consumable electrode portions are made larger than the active areas of the permanent electrodes. The anodic face is designed to have a larger area than the respective cathodic face of the same electrode.

Surprisingly, according to the present invention, freedom from attack of the cathdic zones intended to remain inactive has been obtained without the necessity for the.

above-described refractory coating thereon simply by suitably shaping the respective anodic layers of the consumable electrode portions of carbonaceous material. This consumable anodic portion may be a layer of carbonaceous material in the form of a flat plate or of a plurality of strips, and may be replaced after its almost total consumption, or the layer may be continuously integrated into the contiguous permanent electrodeblock. According to the invention, each of the suitable shapes of the consumable electrodes have projections which extend beyond the faces of the permanent electrode portion so that the electrolytically active area of the consumable anode will be larger than the opposed and confronting cathodic electrodic area.

This surprising result, namely freedom from corrosion without the requirement for framing the electrodes in insulating refractory material, may be explained by assuming that the, electrolytic current is compelled, in the structure according to the invention, to pass almost totally through the electrolytically active anodic surfaces. According, only a minimum portion of the current can by-pass the electrode while deviating or turning around the projecting edges of the consumable anode. Thus, practically on electrolytic attack occurs on the electrode sides in the cathodic zone.

It is known that for optimum operation of aluminum electrolytic furnaces, the anode current density is preferably kept low while the cathode current density is kept high. Also, it is known that a large electrodic area corresponds to a low current density, and vice versa.

In the common conventional cells rovided with at cathodic vat and with pre-baked or Soederberg-type anodes, the constructive peculiarities of the cells unavoidably impose an unfavorable ratio, namely smaller than one, between the anodic active area and the cathodic active area. In the types of cells provided with bipolar electrodes recently proposed, for example in De Varda Patent No. 3,029,194 and in the Patent No. 3,178,363 of De Varda et al., the equality between the active areas of the anodic and cathodic active faces was already considered a progress. In such electrodes, the congruency or similar shape of the areas of opposite polarity was considered as grounded on necessity.

These concepts of the prior art and their limitations have been overcome through the present invention. The

choice of an optimum area for the anode and the choice of an optimum area for the cathode are no longer hindered by or dependent upon each other, but are governed, according to the invention, by the following principles:

(a) The anodic activity decreases with an increase in the inter-electrode path of the electrolysis current. This ath between the cathode and anode becomes longer and longer with respect to the normal or perpendicular interelectrodic distance proper, which is measured perpendicularly between the faces of opposite polarity. Consequently, the extent of the active portion of the anodic area is bound proportionally to the extent of the opposed confronting cathodic area, as a function of this electrolysis current path between cathode and confronting anode.

On the other hand, although an inactive border at the projecting edges of the anodic area acts as side protection according to theinvention, the optimum or useful size of such border, which may best be evaluated according to the individual case, will usually be about 10 to 25 centimeters. It would be irrationaLto exceed the useful size of such border.

(b) The size of the consumable electrode portion will be limited by the structural requirements relative to the supporting of the anodic layer on the permanent portion of the electrode bearing it, namely the surface portion onto which the anodic layer is applied.

(c) The difference in area between the opposed electrodes of opposite polarity will result in a difference in current density, and thus affect the temperature gradient, which causes an ascensional flow of the bath in the im mediate vicinity of the active cathodic face. The area of the electrodes and operation of the furnace should be controlled so that the ascensional or upward vflow of the bath determined in the immediate vicinity of the active cathodic face does not attain such a magnitude as to cause dithculty in the descent of the aluminum which separates out on the cathodic face.

It is another object of the invention to maintain the active anodic area of the electrodes in an electrolytic cell profitably larger than the active cathodic area thereof, and to maintain such greater relationship during at least the major portion of the electrolysis operation.

It is a further object of the invention to substantially reduce the amount ofcorrosion of the electrolysis operation and bath on the surfaces of the electrodes other than those facing an electrode of opposite polarity.

Further objects, advantages and features of the invention, said features being set forth with particularity in the claims annexed hereto, will be apparent from and will be mentioned in the following description with reference to several embodiments according to the invention illustrated by way of example in the accompanying drawing, in

which:

FIG. 1 illustrates diagrammatically a top view of a portion of a multicell furnace with hanging electrodes and with the furnace top removed;

FIGS. 2A, 2B, 2C, 2D, 2E, and 2F each diagrammatically illustrate in top view other respective variants in the shapes of the sides of bipolar electrodes and of the joints between the consumable and permanent portions thereof;

FIG. 3A is a longitudinal vertical section taken along the line IIIAIIIA of FIG. 1, showing another variant of a bipolar electrode;

FIG. 3B is a longitudinal vertical section taken along the line IIIB-IIIB of FIG. 1, showing another bipolar electrode according to a further variant.

FIG. 4 illustrates in vertical section and schematically a piston device for lowering and integrating the consumable electrode of FIG. 3B in the permanent electrode portion; and

FIG. 5 is a cutaway perspective view of a furnace according to FIG. 1 and containing bipolar electrodes according to FIG. 3B.

For purposes of clarity, and to avoid obstructing the drawing, the furnace lid has been omitted in each of the only the top surfaces of the electrodes have been indicated, and the sloping electrode sides have not been indicated in these views.

The same numerals designate the same or corresponding elements throughout the several views.

The structure of the furnace vat is generally of the kind described in the above-mentioned Patent No. 3,178,- 363 of De Varda et a1. As best shown in FIGS. 1 and 5, intermediate bipolar electrodes are arranged between terminal or end-electrodes 1 and 2'. The bipolar electrodes 3 are rigidly suspended from supporting bars 5 fastened to longitudinal beams 27. The bars 5 are fastened to the beams 27 by fastening members or collars 19. Each bar 5 is electrically insulated from its suspension beam by an insulator 20. The beams 27 are also electrically insulated from the remainder of the furnace.

The terminal (monopolar) electrodes 2, 1, 2 and 1 (FIG. 1) are provided with current-supply bars or stubs 4 44 and 4 The terminal current-supply bars 4 4-4 and 4 of FIG. 1, and also the current-supply connecting bars 28, located at the right extremity (FIG. 5) of the necklace furnace, serve to suspend said terminal electrodes.

The monopolar end electrodes, namely terminal anode 1 and terminal cathode 2 shown in FIG. 1, as well as the heading intermediate electrodes, namely cathodic electrode 2 and anodic electrode 1, and the bipolar intermediate electrodes 3, are all respectively suspended by means of the current-conveying stubs 4 (positive), 4 (negative), and 4, and by means of the suspension stubs or bars 5.

The internal surface of walls 6 and bottom walls 7 of the furnace are covered with a refractory layer 8, re sistant to corrosion by the bath and by the molten aluminum. The layer 8 is electrically insulating, preferably of silicon-nitride-bonded silicon carbide or other material with equivalent qualities.

A central wall 9, provided with collecting pockets 10 for receiving the molten aluminum produced, is made of a similar refractory material resistant to the molten cryolitic bath and to the molten aluminum.

As shown in FIG. 5, the pockets 10 are connected through a conduit 29 with the groove 25 of the inclined bottom. An overflow Weir 53 serves to let the molten aluminum overflow into a receptacle 51 common to each series of cells. The aluminum collected in said receptacle can be poured, i.e. removed at any time during the furnace operation upon removing the receptacle lid.

As shown in the drawings, the essential features of the invention can be recognized wherein the permanent portions 3 and 3a3i of the bipolar electrodes and of the terminal electrodes possess a reduced width with respect to the widths of the consumable anodes 11 and Ila-11f.

Also, as may be seen, for example, from FIGS. 3A and 3B, the overall height or depth of the less wide elec trodes 3 and 3a-3i is smaller with respect to that of the consumable anodes 11 and 11a11i. Not only the bipolar electrodes, but also the permanent portions of the terminal anodes have such reduced dimensions of width and depth, as is best seen in FIG. 1.

A variant of the shape of the consumable electrode portion, namely with chamfered edges 31, is indicated at 11i, for example, in FIG. 1. The electrodes illustrated by way of example are all without refractory frames or coatings at their sides and bottoms.

FIGS. 2A through 2F illustrate six respectively different shapes of the suspended bipolar electrodes a-f as compared to the bipolar intermediate electrode 3 represented in FIG. 1.

In the a type bipolar electrode shown in FIG. 2A, the permanent electrode 3a does not change its shape as compared to 3 of FIG. 1, but the consumable anode 11a has borders tapered inwardly towards the rectangular prismatic permanent electrode 3a, at the sides as well as at the bottom of 11a, the bottom being similar to the bottom of the anode 11g of FIG. 3A.

On the other hand, in the b type of electrode shown in FIG. 2B, the electrodic block jointly formed by permanent electrode 3b and its consumable anode 11b together possess a trapezoidal shape, sloping on the sides and on the bottom at an angle which makes the active anodic face 32 of consumable electrode 11b proportionately larger than the cathodic face 33 of electrode 3b in ac cordance with the anode consumption to be expected caused by the by-passed current.

The c type electrode of FIG. 2C has a trapezoidal shaped permanent portion 30 and trapezoidal shaped consumable portion 110, with sides having respectively different angles of slope. The same is true for the 2 type of electrode shown in FIG. 2E, the difference between the c and e types being that the planes of the lateral surfaces of the consumable and permanent electrodes in the case of the 0 type forms an angle greater than a 180, while in the e type the corresponding angle is less than 180. In each case the active areas of the two electrode portions are adjusted in accordance with the anode consumption to be expected due to the by-passed current.

Similarly, other electrode shapes according to the invention are illustrated in the types d and f, illustrated in FIGS. 2D and 2F, in which the permanent electrodes are designated by 3d and 3i, and the consumable electrodes are designated as 11d and 11 Many other shapes of the respective permanent and consumable electrode portions are also conceivable within the scope of the present invention. 7

In the f type of electrode illustrated in FIG. 2F, the consumable electrode 11 has its lateral and bottom end portions projecting towards the right, as viewed in the illustration, forming an indentation which serves to hold the consumable anode 11 in place laterally and inferiorly with respect to the permanent electrode 3 which then needs no such top support as 14 in FIG. 3A.

, The same is true in the embodiment of FIG. 2D, where the backwardly sloping lateral edges form an indentation in the consumable anode 11d, into which the permanent electrode 3d rests to hold the consumable anode 11d in lace. p FIGS. 3A and 3B illustrate two further types of suspended bipolar electrodes according to the invention. The electrode of the g type of FIG. 3A comprises an anodic consumable plate 11g, which may be either in one piece or in the form of individual adjacent vertical or horizontal consumable strips of carbonaceous material, adhering to the permanent electrode 3g. Due to the inclination of the electrode, the buoyancy or upward hydrostatic thrust of the bath forces the consumable anode against the permanent electrode 3g, and causes adhesion, even when the consumable portion is in the form of strips.

The plate 11g of electrodic carbon, less than 12 cm. thick and preferably about 4 cm. thick, and of an appropriate width wider than the face of the permanent electrode to be covered, is applied against said face. Such a plate, of the appropriate shape, rectangular for instance, and measuring, for example, x 70 cm., is slipped into the gap between the electrodes forming the cell and made to adhere to the anode in such a way that, during cell operation, the current will not meet an excessive resistance in passing through the separating layer between the two electrode portions.

Instead of integrating the anode by means of one single plate, it may be advisable to prepare in advance a number of rectangular strips, as described in the abovementioned Patent 3,029,194 of De Varda, all these strips being equally thick, to be juxtaposed onto the anodic face, and so dimensioned as to provide the projection according to the invention.

The permanent electrode 3g of FIG. 3A is suspended by means of the suspension bars 5, and its upper portion is protected by a thick coating or layer 12 of refractory material resistant to the bath and electrically insulating. The layer 12 thus also protects the suspension bars 5 from corrosive attack of the bath. As shown in FIGS. 3A and 3B, the level 13 of the electrolytic bath preferably reaches about the middle of the vertical height of the refractory layer 12; However, this latter coating 12 may also be dispensed with if the upper portion of the electrode is maintained above the surface of the bath.

, A second layer 14 of common refractory, superimposed on the layer 12, serves as an upper stopping member and abutment for the consumable and interchangeable anodic plate or strips 11g. If the plate is held in place by bottom indentation, layer 14 is no more required.

The wedge-shaped type of electrode h shown in FIG. 3B allows a continuous or semi-continuous integration of the consumable anode 1111. In this embodiment, the consumable anode has a triangular longitudinal vertical section. Movement of the electrode 1111 is performed through a feeding stack 15, preferably built of refractory material and mounted above the electrode 311. The arrow P indicates the direction of feed of the anodic material for making up for the electrolytic consumption of the electrode 311.

Aswill be noticed from FIGS. 3B and 4 as compared to FIG. 3A, the 12 type of consumable electrode 11h has a slightly different shape as compared to that of the g type of FIG. 3A, the h type being wedge-shaped. In the h type it is also possible, but not necessary, to protect the permanent electrode at the top with a special refractory coating 16 having the same characteristics as coating 12 of FIG. 3A.

Of course, the feeding (in the direction of arrow P) of the anodic material, in the shape of a plate or of prebaked strips, or of Soederberg anodic paste as the case may be, should not be so great as to thrust the integrable anode immersed in the bath too far beyond the lower border. of the permanent anode 311, in order thatit does not contact the metallic layer of aluminum which is produced and which collects in the channel 17 below as provided in the furnace bottom 7 and communicating with the pocket 10.

In the case where the formation of electrode 1111 is by means of feeding Soederberg paste, the feeding stack 15 is preferably lined inside with a metallic jacket 15a (FIG. 4) in order to allow a sufiicient sliding of the paste material which is fed downwards in the direction of arrow P in accordance with the amount of the electrolytic consumption. The feeding is performed by means of a piston device, schematically illustrated in FIG. 4, and consisting of a gear 33, cooperating with a rack 34 and a piston 35 which reciprocates within the jacket 15: above the paste which forms the electrode 11h, baking into a hardened anode as the electrolysis proceeds.

Arrow B in FIG. 5 indicated the direction of flow of the slowly moving cryolit-ic bath, and the arrow E indicates the electric current direction. The vat 6 may be protected on the outside by an insulating jacket 43, providing thermal insulation.

The refractory frames provided in the multicell electrolytic furnaces with suspended electrodes according to the above-mentioned PatentNo. 3,178,363 of De Varda et al. act in their bottom-covering portion also as bottom stops or abutments for the consumable anodic plates or strips. According to the present invention the consumable plates or strips such as 11g are kept in place by the buoyancy or hydrostatic pressure of the bath, namely the thrust component of the bath pressure, acting perpendicular to the active surface of the plates or strips and in cooperation with either an upper bearing plane or layer 14 of refractory above the bath surface and over the consumable electrode portion 11g, or in cooperation with the feeding pressure P (FIGS. 3B, 4) of the anodic material.

itself in the case of continuous or semi-continuous feeding thereof, or by the bottom indentation of the anodic plate.

As is customary in other multicell electrolytic furnaces, the furnaces according to the present invention are accurately closed in the upper part in order to prevent the infiltration of any undesired air and to assure a suflicient thermal insulation.

FIGS. 2 and 3 illustrate only the bipolar electrodes. The permanent terminal or end electrodes 1, 1', 2 and 2' are not illustrated in these figures, but in all cases they will have a shape and size corresponding to those illus-v trated in FIG. 1. The bipolar electrodes 3 and 3zz-3i, whatever the shape adopted, will all preferably be the same in the same electrolysis furnace. Accordingly, it is understood that the consumable anode variation 11i illustrated with chamfered edges in FIG. 1 is only for illustration of a different embodiment, and to reduce the number of figures required.

The relative sizes of the active electrode areas and their ratios have already been discussed hereinabove. Typical examples of these values are as follows:

A suitable ratio between active cathodic face area and active anodic face area for a normal inter-electrodic distance in the order of magnitude of 5 cm. (measured perpendicularly between the faces of opposite polarity) will be equal to or smaller than 0.95, preferably between 0.9 and 0.3. This would correspond to a structure having border projections of the anodic sides beyond the cathodic sidesof 5-50 cms. per side, and preferably of 10-25 cms. per side,.on at least three sides, that is, the two lateral flanks and the bottom side.

The lateral sides or flanks as well as the bottoms of the permanent electrodic surfaces should not be electrolytically active, and the structure of the present invention aims to achieve this goal. Even if the by-pass current causes a consumption of slight magnitude at the flanks or bottoms of the permanent electrodes 3 and Eta-3f, nevertheless the operation of such permanent electrodes is secured for a sufficiently long period as to allow the furnace run to be considered economical.

Should it be deemed necessary to then restore the original shape and dimensions of the permanent electrodes, the consumption thereof can be compensated for, after removing them from the bath and levelling their surfaces, by applying thereon integration sheets of the same carbonaceous material with the aid of a layer of binder, for instance of pitch, by the operation known, per se, from the above mentioned Patent No. 3,029,194.

Moreover, the protruding borders of the abovedescribed consumable anodic plates or strips 11 may be shaped in accordance with the dilferent extent of electrolytic consumption of said borders. Similarly, the permanent electrodes 3 may also have a shape making up at least partially for a possible modest consumption thereof. This adjusting of the shape of the respective electrode portions to the expected consumption is illustrated, for example, in FIGS. 2B, 2C and 2E.

Among the advantages achieved by the present invention, wherein the active anodic area is made economically to predominate over the active cathodic area, it is possible to maintain this greater predominance of area for a relatively long time during the electrolysis operation, owing again to the rather high reduction of the amount of corrosion on the sides of the electrodes.

It will be understood by those skilled in the art from the foregoing that variants other than those illustrated are possible, Within the concepts of the present invention as defined in and within the scope of the claims appended hereto. Thus, for instance, the flanks and bottoms of the permanent. portions 3 of the bipolar electrodes, instead of being entirely blank, in certain cases for additional protection, these electrodes may be either partly or entirely covered with inert material for protection against attack by the bath, without departing from the scope of the present invention.

I claim:

1. A furnace for electrolytic production of aluminum, comprising a vat adapted to contain a fluoride bath of dis: solved alumina, a series of electrodes rigidly suspended in said vat including a plurality of terminal electrodes and at least one bipolar intermediate electrode between said terminal electrodes, consecutive electrodes of said series defining mutually spaced faces of contrary polarity, said bipolar intermediate electrode comprising a permanent portion with a face having lateral and bottom edges and a consumable portion of respectively different polarity, said consumable portion having projections extending beyond a plurality of the cdgesof the faces of said permanent portion so that the electrolytically active area of said consumable portion is larger than the electrolytically active area of the opposed and confronting electrodic face of the next adjacent electrode of said series.

2. A furnace according to claim 1, said terminal electrodes including at least one terminal electrode having a permanent portion and a consumable anodic portion, said consumable portion of said terminal electrode having at least one dimension of width or depth greater as compared to the dimensions of said permanent portion of the same electrode.

3. A furnace for electrolytic production of aluminum, comprising a vat adapted to contain a fluoride bath of dissolved alumina, a series of electrodes, suspension means rigidly supporting said electrodes from above in said vat, said electrodes including a plurality of terminal monopolar electrodes and a plurality of bipolar intermediate electrodes between said terminal electrodes, said bipolar intermediate electrodes having a first portion and a second portion of opposite polarity contiguous with said first portion, a header of protective refractory material fixedly attached to said first portion, said second portion comprising a separate layer of carbonaceous material capable of being replaced during the electrolytic operation, said second portion having at the interface joining said portions a contour defining lateral and bottom edges projecting beyond the contour of said first portion.

4. A furnace for electrolytic production of aluminum, comprising a vat adapted to contain a fluoride bath of dissolved alumina, a series of electrodes rigidly suspended from above in said vat, said electrodes including a plurality of terminal electrodes and at least one bipolar intermediate electrode between said terminal electrodes, consecutive electrodes of said series defining mutually spaced faces of contrary polarity, said bipolar intermediate electrodes each comprising a permanent cathodic portion and a consumable anodic portion, said anodic portion and said cathodic portion defining respective electrolytically active faces oriented in opposite directions, the electrolytically active face of said consumable anodic portion having lateral and bottom edge portions extending beyond the electrolytically active face of said permanent cathodic portion, the ratio between the active cathodic area and active anodic area of a respective bipolar electrode not exceed ing 0.95. v

5. A furnace according to claim 4, said ratio between the active cathodic area and active anodic area of a respective bipolar electrode being between 0.9 and 0.3.

6. A furnace for electrolytic production of aluminum, comprising a vat adapted to contain a fluoride bath of dissolved alumina, a series of electrodes rigidly suspended from above in said vat, said electrodes including a plurality of terminal electrodes and at least one bipolar intermediate electrode between said terminal electrodes, consecutive electrodes of said series defining mutually spaced faces of contrary polarity, said bipolar intermediate electrodes each comprising a permanent cathodic portion and a consumable anodic portion, said anodic portion and said cathodic portion defining respective electrolytically active faces oriented in opposite directions, the ratio between the active cathodic area and active anodic area of a respective bipolar electrode not exceeding 0.95, said anodic portion having edges projecting beyond said cathodic portion to define a projecting border at each edge of between and 50 cm.

7. A furnace for electrolytic production of aluminum, comprising a vat adapted to contain a fluoride bath of dissolved alumina, a series of electrodes rigidly suspended from above in said vat, said electrodes including a plurality of terminal electrodes and at least one bipolar intermediate electrode between said terminal electrodes, consecutive electrodes of said series defining mutually spaced faces of contrary polarity, said bipolar intermediate electrodes each comprising a permanent cathodic portion and a consumable anodic portion, said anodic portion and said cathodic portion defining respective electrolytically active faces oriented in opposite directions, the ratio between the active cathodic area and active anodic area of a respective bipolar electrode not exceeding 0.95, said anodic portion having edges projecting beyond at least the sides and bottom of said cathodic portion an amount of between 10 and 25 cm. per edge.

8. A furnace for electrolytic production of aluminum, comprising a vat adapted to contain a fluoride bath of dissolved alumina, a series of electrodes rigidly suspended from above in said vat, said electrodes including a plurality of terminal electrodes and at least one bipolar intermediate electrode between said terminal electrodes, consecutive electrodes of said series defining mutually spaced electrolytically active faces of contrary polarity, said bipolar intermediate electrodes each comprising a permanent cathodic portion and a consumable anodic portion, said anodic portion having width and length greater than corresponding parallel dimensions of said cathodic portion so as to make the electrolytically active face of said anodic portion greater than the electrolytically active face of said cathodic portion of the same bipolar electrode.

9. A furnace according to claim 8, the electrolytically active anodic area of the consumable portions of said bipolar electrodes being greater than the electrolytically active cathodic areas of the faces spaced from said confront ing same.

' 10. A furnace according to claim 8, said consumable portion and said permanent portion having sides lying in respective planes which form an angle with each other.

11. A furnace for electrolytic production of aluminum, comprising a vat adapted to contain a fluoride bath of dissolved alumina, a series of electrodes rigidly suspended from above in said vat, said electrodes including a plurality of terminal electrodes and at least one bipolar in termediate electrode between said terminal electrodes, consecutive electrodes of said series defining mutually spaced electrolytic-ally active faces of contrary polarity, said bipolar intermediate electrodes each comprising a permanent cathodic portion and a consumable anodic portion, said anodic portion having at least one dimension greater than a corresponding parallel dimension of said cathodic portion so as to make the electrolytically active face of said anodic portion greater than the electrolytically active face of said cathodic portion of the same bipolar electrode, said consumable portion and said permanent portion havin g bottoms lying in respective planes which form an angle with each other.

12. A furnace according to claim 8, said consumable portion and said permanent portion having edges which form a step with each other.

13. A furnace for electrolytic production of aluminum, comprising a vat adapted to contain a fluoride bath of dissolved alumina, a series of electrodes rigidly suspended from above in said vat, said electrodes including a plurality of terminal electrodes and at least one bipolar intermediate electrode between said terminal electrodes, consecutive electrodes of said series defining mutually spaced electrolytically active faces of contrary polarity, said bipolar intermediate electrodes each comprising a permanent cathodic portion and a consumable anodic portion, said anodic portion having at least one dimension greater than a corresponding parallel dimension of said cathodic portion so as to make the electrolytically active face of said anodic portion greater than the electrolytically active face of said cathodic portion of the same bipolar electrode, said consumable portion defining a recessed indentation in which said permanent portion is disposed.

14 A furnace for electrolytic production of aluminum, comprising a vat adapted to contain a fluoride bath of dissolved alumina, a series of electrodes rigidly suspended from above in said vat, said electrodes including a plurality of terminal electrodes and at least one bipolar intermediate electrode between said terminal electrodes, consecutive electrodes of said series defining mutually spaced electrolytically active faces of contrary polarity, said bipolar intermediate electrodes each comprising a permanent cathodic portion and a consumable anodic portion, said anodic portion having at least one dimension greater than a corresponding parallel dimension of said cathodic portion so as to make the electrolytically active face of said anodic portion greater than the electrolytically active face of said cathodic portion of the same bipolar electrode, said consumable portion and said permanent portion each defining a prism of uniform trapezoidal horizontal cross section, the sides of the respective portions lying in planes which meet at an angle.

15. A furnace according to claim 14, said angle between said planes being greater than 180.

16. A furnace according to claim 14, said angle between said planes being less than 180.

17. A furnace for electrolytic production of aluminum, comprising a vat adapted to contain a fluoride bath of dissolved alumina, a series of electrodes rigidly suspended from above in said vat, said electrodes including a plurality of terminal electrodes and at least one bipolar intermediate electrode between said terminal electrodes, con secutive electrodes of said series defining mutually spaced electrolytically active faces of contrary polarity, said bipolar intermediate electrodes each comprising a permanent cathodic and a consumable anodic portion, said anodic portion having an electrolytically active face area greater than the electrolytically active face area of the cathodic face spaced from and confronting same, said anodic face area including lateraly and bottom area portions extending beyond the area of the cathodic face, said cathodic portion having lateral carbonaceous surfaces in addition to those confronting an anodic face arranged to be in direct contact with said bath and free from refractory coating thereon.

18. A furnace for electrolytic production of aluminum, comprising a vat adapted to contain a fluoride bath of dissolved alumina, a series of electrodes rigidly suspended in said vat including a plurality of terminal electrodes and at least one bipolar intermediate electrode between said terminal electrodes, consecutive electrodes of said series.

defining mutually spaced faces of contrary polarity, said bipolar intermediate electrode comprising a permanent portion with a face having lateral and bottom edges and a consumable portions of respectively different polarity,

said consumable portion having projections extending beyond a plurality of the edges of the faces of said permanent portion so that the electrolytically active area of said consumable portion is larger than the electrolytically active area of the opposed and confronting electrodic face of the next adjacent electrode of said series, at least one of said terminal electrodes having a cathodic surface, the cathodic surfaces of each of said electrodes including that of said one terminal electrode being mutually equal.

19. In a process of producing aluminum by electrolysis of alumina in a fused salt bath in which an electric current is passed serially through a solid anodic surface, an intervening electrolysis gap of fused bath, through anintervening intermediate bipolar solid electrode providing opposite anode and permanent cathode polar surfaces, a second electrolysis gap of said bath, and eventually through a permanent cathodic surface, the anodic surfaces being consumable in the electrolysis, the improvement comprising replacing the consumed anodic surface during the electrolysis operation with carbonaceous material so as to form a polar surface greater in width and length than corresponding paralleldimensions of the permanent polar surface opposed thereto, and so that the electric current defines a cathodic current density greater than the corresponding anodic current density.

References Cited UNITED STATES PATENTS 484,416 10/1892 Fletcher 204268 2,480,474 8/1949 Johnson 204268 X 2,938,843 5/1960 De Varda 204-67 2,952,592 9/1960 De Varda 20467 2,959,527 11/1960 De Varda 204-67 3,029,194 4/1962 De Varda 20467 3,133,008. 5/1964 De Varda 20467 X 3,178,363 4/1965 De Varda et al. 20467 FOREIGN PATENTS 561,017 7/1958 Canada.

JOHN H. MACK, Primary Examiner.

G. KAPLAN, Assistant Examiner. 

1. A FURNACE FOR ELECTROLYTIC PRODUCTION OF ALUMINUM, COMPRISING A VAT ADAPTED TO CONTAIN A FLUORIDE BATH OF DISSOLVED ALUMINA, A SERIES OF ELECTRODES RIGIDLY SUSPENDED IN SAID VAT INCLUDING A PLURALITY OF TERMINAL ELECTRODES AND AT LEAST ONE BIPOLAR INTERMEDIATE ELECTRODE BETWEEN SAID TERMINAL ELECTRODES, CONSECUTIVE ELECTRODES OF SAID SERIES DEFINING MUTUALLY SPACED FACES OF CONTRARY POLARITY, SAID BIPOLAR INTERMEDIATE ELECTRODE COMPRISING A PERMANENT PORTION WITH A FACE HAVING LATERAL AND BOTTOM EDGES AND A CONSUMABLE PORTION OF RESPECTIVELY DIFFERENT POLARITY, SAID CONSUMABLE PORTION HAVING PROJECTIONS EXTENDING BEYOND A PLURALITY OF THE EDGES OF THE FACES OF SAID PERMANENT PORTION SO THAT THE ELECTROLYTICALLY ACTIVE AREA OF SAID CONSUMABLE PORTION IS LARGER THAN THE ELECTRICALLY ACTIVE AREA OF THE OPPOSED AND CONFRONTING ELECTRODIC FACE OF THE NEXT ADJACENT ELECTRODE OF SAID SERIES.
 19. IN A PROCESS OF PRODUCING ALUMINUM BY ELECTROLYSIS OF ALUMINA IN A FUSED SALT BATH IN WHICH AN ELECTRIC CURRENT IS PASSED SERIALLY THROUGH A SOLID ANODIC SURFACE, AN INTERVEINING ELECTROLYSIS GAP OF FUSED BATH, THROUGH AN INTERVEINING INTERMEDIATE BIPOLAR SOLID ELECTRODE PROVIDING OPPOSITE ANODE AND PERMANENT CATHODE POLAR SURFACES, A SECOND ELECTROLYSIS GAP OF SAID BATH, AND EVENTUALLY THROUGH A PERMANENT CATHODIC SURFACE, THE ANODIC SURFACES BEING CONSUMABLE IN THE ELECTROLYSIS, THE IMPROVEMENT COMPRISING REPLACING THE CONSUMED ANODIC SURFACE DURING THE ELECTROLYSIS OPERATION WITH CARBONACEOUS MATERIAL SO AS TO FORM A POLAR SURFACE GREATER IN WIDTH AND LENGTH THAN CORRESPONDING PARALLEL DIMENSIONS OF THE PERMANENT POLAR SURFACE OPPOSED THERETO, AND SO THAT THE ELECTRIC CURRENT DEFINES A CATHODIC CURRENT DENSITY GREATER THAN THE CORRESPONDING ANODIC CURRENT DENSITY. 